A conductance catheter includes a plurality of electrodes spaced along an elongated portion that is placed in a biological chamber. A current is imposed between the outermost electrodes, and the potentials at the intermediate electrodes are sensed. The conductance of each volume segment between adjacent electrodes is determined by dividing the current by the change in voltage, and the volume of each segment is linearly related to conductance through it. There is a parallel conductance of electricity through the wall of the chamber and the surrounding tissue, and this parallel conductance causes the volume to be overestimated.
Baan, J. et al., "Continuous stroke volume and cardiac output from intraventricular dimensions obtained with impedance catheter," Cardiovasc, Res. 15: 328-334, 1981, discloses using a conductance catheter to measure stroke volume by sensing the difference in impedance between the beginning and end of the ejection.
Baan, J., et al., "Continuous measurement of left ventricular volume in animals and humans by conductance catheter," Circulation 70: 812-823, 1984 ("Baan et al. 1984"), discloses correcting for parallel conductance by determining an offset correction term, V.sub.c, which is determined by either temporarily reducing the volume to zero by suction or by injecting a bolus of cold glucose or hypertonic saline into the chamber, the latter approach also being employed by Burkhoff, D., et al., "Accuracy of volume measurement by conductance catheter evaluated in isolated, ejecting canine hearts", Circulation, 72: 440-447, 1985 ("Burkhoff et al. 1985") and Carlson, Drew, E. et al., "Right atrial volume during hemorrhage in the dog," Am. J. Physiol., H1136-H1144, 1986.
Salo U.S. Pat. No. 4,674,518 discloses a conductance catheter using two frequencies at two pairs of drive electrodes having different distances between them. The impedances resulting from different frequencies are plotted to extrapolate the impedance value for infinitely spaced electrodes, and this impedance value is used to determine stroke volume.
McKay, Raymond, G. et al., "Instantaneous measurement of left and right ventricular stroke volume and pressure-volume relationships with an impedance catheter," Circulation, 69: 703-710, 1984, discloses using a 1.3 kHz frequency to drive a conductance catheter, owing to the increased resistivity (and thus smaller parallel conductance) of myocardial tissue at 1.3 kHz vis-a-vis resistivity at 20 kHz.